1. Field of the Invention
The present invention relates to an image supporting sheet, and more particularly to an image supporting sheet for sublimation thermal transfer recording wherein a recorded image has excellent resistance to heat, light, solvents, scratch and alteration.
2. Discussion of Background
Recently, the demand for full color printing has been increasing year by year. There have been known recording methods for full color printing including the electrophotographic method, the ink jet method and the thermal transfer method. Among these methods, the thermal transfer method is most widely employed because of its advantages such as easy maintenance and low noise operation.
The thermal transfer recording methods can be roughly classified into two types, a thermofusing thermal transfer recording and a sublimation thermal transfer recording.
In the thermofusing thermal transfer recording, an image can be obtained by applying heat to the backside of a thermofusing thermal transfer recording sheet superimposed on a image receiving sheet. The thermofusing thermal transfer recording sheet has a substrate and an ink layer, formed on the substrate, which has a coloring agent dispersed in a thermofusible material, and when applying heat to form an image, the ink layer melts and transfers to the image receiving sheet, so that an image is obtained.
In the sublimation thermal transfer recording, an image can be also obtained by applying heat to the backside of sublimation thermal transfer recording sheet superimposed on a dye receiving layer formed on a image receiving sheet. The sublimation thermal transfer recording sheet has a substrate and an ink layer, formed on the substrate, which has a thermo-diffusional dye (hereinafter a sublimable dye) dispersed in a binder resin, and when applying heat to form an image, the sublimable dye diffuses into the dye receiving layer, so that an image is obtained.
When these two recording methods are compared for full color printing, the sublimation thermal transfer recording is superior to the thermofusing thermal transfer recording because of advantages such as high resolution and high fidelity of color tone and half tone.
However, the sublimation thermal transfer recording method costs more to run than these other methods, because:
(a) a sublimable dye is very expensive;
(b) yellow, magenta, cyan, and, when necessary, black image transfer recording sheets, each individually being of equal size to the recorded image, are needed to obtain a full color image; and
(c) a used sublimation thermal transfer recording sheet must be disposed even though there is a large unused part of the recorded sheet.
To eliminate this shortcoming, the so-called multiple sublimation thermal transfer recording methods have been proposed. The multiple sublimation thermal recording methods include the n-times (n&gt;1) mode recording method and the n-fold speed mode recording method.
The n-times mode recording method means that a sublimation thermal transfer recording sheet is repeatedly printed n-times under the condition of the same printing speed as an image receiving sheet.
The n-fold speed mode recording method means that a sublimation thermal transfer recording sheet is printed under the condition of the 1/n printing speed to an image receiving sheet.
An image printed by the n-fold speed mode recording method is superior to an image printed by the n-times mode recording method because of advantages such as good evenness of the image and no wrinkling of the thermal transfer recording sheet in printing.
In the n-fold speed printing, since the sublimation thermal transfer recording sheet and the image receiving sheet are run at different speeds in printing, a strong friction force occurs between the sublimation thermal transfer recording sheet and the image receiving sheet, so that the two sheets tend to stick to each other or are damaged. Accordingly, the image receiving sheet used in n-fold speed mode printing is required to be more resistant to heat of printing and more lubricating to the sublimation thermal transfer recording sheet than in n-times mode printing. To address this problem, it has been proposed to include a releasing agent, such as a silicone oil and a silicone resin in the dye receiving layer of the image receiving sheet.
In other than n-fold speed mode printing, namely, in the case of normal sublimation printing, a releasing agent has also been employed to a dye receiving layer to improve releasability from the sublimation thermal transfer recording sheet. (JP-A 64-44781)
Furthermore, the sublimation thermal transfer recording has a shortcoming in that the printed image tends to change color or fade away by heat and light, particularly, ultraviolet rays. Recently, an image printed by the sublimation thermal transfer recording method has also been used as a recording material for identification cards, so that it is necessary to prevent the image from alteration. To address this subject, it has been proposed that an image printed by sublimation thermal recording method be covered or laminated by a clear protective film.
However, it is difficult to cover or laminate a clear protective film on an image receiving sheet because the dye receiving layer of the receiving sheet which includes a releasing agent such as a silicone oil and a silicone resin has a very low surface energy.
Unfortunately, due to all of these reasons, a need exists for an image supporting sheet, printed by a sublimation thermal transfer recording method, wherein a recorded image has excellent resistance to heat, light, solvents, scratch, and alteration.